Hydroelectric power, or hydroelectricity, is basically electrical energy that has been generated using natural forces such as gravity or flowing water. It's usually produced by dams, because dams can store and direct large volumes of water. Hydroelectric power is becoming increasingly popular.
Dams can generate electricity because they contain special mechanisms designed to take the energy in flowing water and turn it into electrical power. A power source is used to spin a turbine—basically a propeller—which in turn spins a metal shaft, forming the equivalent of an electrical generator that produces and stores electricity. The power source, in the case of hydropower, is water itself. When water moves a turbine, the turbine spins, and electromagnets in the turbine generate an electric current in the stationary coils of wire inside them.